1. Field of the Invention
The invention relates generally to systems for drilling boreholes for the production of hydrocarbons. More particularly, the invention relates to devices and methods for protecting the bit during the initial stages of the drilling operation in order to extend the lifetime of the bit.
2. Description of Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. Modem drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at end thereof that is rotated by a drill motor (mud motor) and/or the drill string. Pressurized drilling fluid (commonly known as the “mud” or “drilling mud”) is pumped into the drill pipe to rotate the drill motor and to provide lubrication to various members of the drill string including the drill bit. The drill pipe is rotated by a prime mover, such as a rotary table, to facilitate directional drilling and to drill vertical boreholes.
Boreholes are usually drilled along predetermined paths and the drilling of a typical borehole proceeds through various formations. The drilling operator typically controls the surface-controlled drilling parameters, such as the weight on bit, drilling fluid flow through the drill pipe, the drill bit rotational speed (rpm of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid to optimize the drilling operations. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to optimize the drilling operations. For drilling a borehole in a virgin region, the operator typically has seismic survey plots that provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator also has information about the previously drilled boreholes in the same formation. Additionally, various downhole sensors and associated electronic circuitry deployed in the BHA continually provide information to the operator about certain downhole operating conditions, condition of various elements of the drill string and information about the formation through which the borehole is being drilled.
Typically, the information provided to the operator during drilling includes drilling parameters, such as WOB, rotational speed of the drill bit and/or the drill string, and the drilling fluid flow rate. In some cases, the drilling operator is also provided selected information about bit location and direction of travel, bottomhole assembly parameters such as downhole weight on bit and downhole pressure, and possibly formation parameters such as resistivity and porosity. Typically, regardless of the type of the borehole being drilled, the operator continually reacts to the specific borehole parameters and performs drilling operations based on such information and the information about other downhole operating parameters, such as bit location, downhole weight on bit and downhole pressure, and formation parameters, to make decisions about the operator-controlled parameters.
During the initial part of a drilling operation, the bit is prone to damage. The BHA must be set down into the formation to be drilled as rotation of the bit is begun. Typically, the driller does this manually. As such, the setting down process may be performed differently each time drilling is begun. If setting down and rotation is begun too quickly, the bit may be damaged by the suddenness of the contact with the rock, or the drill string may become overtorqued. If setting down and rotation are done too slowly, rig time is wasted. This is especially true for a new bit, wherein it must be “drilled in” to establish a new pattern. Also, in reaming operations the resumption of drilling with optimal drilling parameters when a re-introduced bit contacts hole bottom should be commenced as soon as possible.
A few systems have been proposed for automated operation of portions of a drilling operation. In general, such systems establish a set point for WOB, and then control the drilling equipment to reach the setpoint quickly. This may be counterproductive. Attempting to achieve the setpoint quickly may cause a step-change to the system that results in damage to the bit, overtorquing of the drill string and other problems.
U.S. Pat. No. 4,875,530 issued to Frink et al., for example, describes an automatic drilling system wherein a required speed and bit weight is input into the system by an operator. A controller device electronically senses the weight on bit and provides instantaneous feedback of a signal to a hydraulically driven drawworks which is capable of maintaining precise bit weight throughout varying penetration modes. Frink's system provides a setpoint for the bit weight. However, Frink also seeks to achieve the setpoint quickly and without regard to protection of the bit.
U.S. Pat. No. 6,382,331 issued to Pinckard describes a method and system for optimizing the rate of bit penetration while drilling. Pinckard's arrangement collects information on bit rate of penetration, weight on bit, pump or standpipe pressure, and rotary torque data during drilling. This information is stored in respective data arrays. Periodically, the system performs a linear regression of the data in each of the data arrays with bit rate of penetration as a response variable and weight on bit, pressure, and torque, respectively, as explanatory variables to produce weight on bit, pressure, and torque slope coefficients. The system calculates correlation coefficients for the relationships between rate of penetration and weight on bit, pressure, and torque, respectively. The system then selects the drilling parameter with the strongest correlation to rate of penetration as the control variable. Pinckard's system, however, does not attempt to solve the problems associated with the start of drilling or drilling in of a bit.
There is a need for a system that overcomes the problems associated with prior art systems as regards the starting of drilling, and, in particular situations, resuming drilling after reaming.